Method of preparing copper for use in the arcing electrodes of a vacuum circuit interrupter

ABSTRACT

A method of preparing copper for use in the electrodes of a vacuum-type circuit interrupter comprising the steps of: melting copper in a vacuum; adding to the molten copper a small amount of magnesium that reacts with any oxygen present in the copper to form magnesia, which floats to the surface of the liquid coppermagnesium solution; maintaining while in a vacuum the temperature of the solution above its melting point for a sufficient time to evaporate from the solution all of the magnesium present therein that has not reacted with the oxygen therein; and then directionally cooling and solidifying said copper to force residual magnesia trapped therein to move to the surface of the liquid copper.

USE IN THE ARCING ELECTRODES OF A VACUUM CIRCUIT INTERRUPTER UnitedStates Patent 1 [111 3,776,719 Foldes Dec. 4, 1973 METHOD OF PREPARINGCOPPER FOR FOREIGN PATENTS OR APPLICATIONS Primary Examiner-W. W.Stallard Attorney-J. Wesley l-Iaubner et al.

[57] ABSTRACT A method of preparing copper for use in the electrodes ofa vacuum-type circuit interrupter comprising the steps of: meltingcopper in a vacuum; adding to the molten copper a small amount ofmagnesium that reacts with any oxygen present in the copper to formmagnesia, which floats to the surface of the liquid copper-magnesiumsolution; maintaining while in a vacuum the temperature of the solutionabove its melting point for a sufficient time to evaporate from thesolution all of the magnesium present therein that has not reacted withthe oxygen therein; and then directionally cooling and solidifying saidcopper to force residual magnesia trapped therein to move to the surfaceof the liquid copper.

4 Claims, 2 Drawing Figures ao/u/va Pom/P A7! ATM.

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I l I I I I I I I 6'0 /00 200 300 400 600 600 M00 M00 2000 3000TEMPE/M70815 "c INVENTOR. STEPHEN F0405,

ATTO/M [Y BACKGROUND This invention relates to a method of preparingcopper for use in the arcing electrodes of a vacuum-type circuitinterrupter.

The electrodes of a vacuum-type circuit interrupter must have a highdegree of freedom from gases and constituents which, upon arcing, arecapable of releasing gases. The gas which is the most difficult toremove is oxygen especially if it is present as a compound.

One method for achieving the required high gas freedom involvesrepetitively zone refining the electrode metal, as disclosed and claimedin U.S. Pat. No. 3,234,35 l-I-lebb, assigned to the assignee of thepresent invention. While quite effective in removing gases, this processis subject to the disadvantage that it is relatively expensive and timeconsuming.

For overcoming this disadvantage, it has been proposed to prepare theelectrode metal by adding thereto, while in the molten stage, smallquantities of zirconium, or titanium, or beryllium. These additives havea high affinity for oxygen and react with the oxygen in the electrodemetal to form highly stable refractory oxides. These oxides float to thesurface of the molten metal and, after the metal cools, can be removedby suitable means, such as etching or machining. Electrodes made bythese processes are disclosed and claimed in U.S. Pat. Nos.3,450,928-Cobine and 3,497,755-Horn, assigned to the assignee of thepresent invention.

In the aforesaid Cobine and Horn patents, the amount of deoxidizingadditive used is made relatively high to insure that all of the oxygenpresent in the base metal is reacted with. This invariably results in anexcess of additive, and this excess forms an alloy with the base metal.Such alloying reduces the conductivity of the base metal and may beotherwise undesirable for certain applications.

SUMMARY An object of my invention is to achieve the required purity andfreedom from oxygen by using a deoxidizing additive, but without leavingbehind any residual amount of the additive for alloying with the copperbase metal.

Another object is to achieve the result of the immediately precedingparagraph within an exceptionally short processing time and with anear-minimum evaporation of copper base metal.

In carrying out my invention in one form, I employ copper as the basemetal and subject this copper to a vacuum melting operation. To themolten copper I add a small amount of magnesium sufficient in quantityto react with all the oxygen present in the copper, to' form magnesiawhich floats to the surface of the coppermagnesium solution. Thereafter,while in a vacuum at a pressure of to 10' torr, the copper-magnesiumsolution is maintained at a temperature level above its melting point,which temperature is far above the boiling point of the magnesium, for asufficiently long period to evaporate all of the magnesium that has notreacted with the oxygen present in the copper. This is followed by adirectional cooling operation which forces any residual magnesiatrapped'in the copper to the surface of the copper.

BRIEF DESCRIPTION OF DRAWING For a better understanding of theinvention, reference may be had to the following description taken inconjunction with the accompanying drawing wherein:

FIG. 1 illustrates apparatus used for practicing a preferred form of theinvention.

' FIG. 2 is a graph showing the vapor pressure v. temperaturecharacteristics of certain metals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1,there is shown a vacuum chamber 10 containing a graphite crucible 12that is suitably supported in a position above and vertically spacedfrom the base plate. The base plate is provided with a suitable fluidpassage 16 for circulating a coolant therethrough when it is desired tocool the base plate.

For heating the metallic charge within crucible 12, an induction heatingcoil 15 is provided about the crucible. When this coil is suitablyenergized by alternating current, the resulting alternating magneticfield acts in a conventional manner to'induce eddy currents within thecharge that produce heating thereof. After the charge has melted inresponse to such heating, the alternating magnetic field imparts astirring action to the molten liquid in the crucible.

In practicing the invention in one form, the crucible is first loadedwith copper of a moderately high purity, such as electrolytic copper orpreferably OFHC copper. The latter type of copperv is obtainable fromAmerican Metal Climax Inc., New York, N.Y. and has been analyzed asincluding approximately 99.995 percent copper generally possessing lessthan 2 parts per million of oxygen. The coil 15 is energized to heat thecopper and convert it into the molten state, in which state it isdepicted in the drawing at 20. The temperature of the copper is raisedby this heating operation to around l,l50 C. Before and during theheating operation, the chamber 10 is evacuated and maintained evacuatedto a high degree in order to prevent contamination of the hot copper bythe atmosphere of the chamber 10.

' When the copper is molten and at a temperature of about l,l50 C, asmall quantity of magnesium is added to the molten copper. Thismagnesium is preferably contained within a small quantity of highlyrefined master alloy of copper and magnesium. This small quantity ofmaster alloy, in the form of a solid mass, is suitably plunged into themolten copper, following which it quickly melts, releasing its magnesiumcontent into the copper 20. The stirring action of the magnetic fieldfrom coil 15 distributes the magnesium throughout the copper.

As the magnesium is thus distributed, it reacts with the oxygen in thecopper to form magnesia, MgO, a highly stable oxide. The magnesia ismuch lighter in weight than the copper, and most of it therefore risesto the top of the molten copper and floats thereon although the magneticstirring will cause some mixing action.

After the magnesium is thus added, the temperature of the charge 20 isheld at about l,l50 C or slightly higher. Since this temperature issubstantially above the boiling point of magnesium, i.e.', about 287 Cat a pressure of 10" torr, the unreacted magnesium that is left in themelt is boiled or evaporated off into the surrounding vacuum. Thetemperature of the melt is maintained at this level for a sufficientlength of time to boil off all of the unreacted magnesium. Onlymagnesia, MgO, is left behind in the copper. Since this boiling takesplace under a high vacuum in chamber 10, i.e., about to 10 torr, themagnesium evaporation is essentially complete and essentially nounreacted magnesium is left behind. After all the unreacted magnesiumhas thus been boiled off, the molten copper in crucible 12 isdirectionally solidified from its bottom up. This is accomplished byreducing the energy input into the coil 15, lowering crucible 12 ontothe base plate 14, and by supplying coolant to passage 16 to cool thebase plate 14. This produces a cooling action at the bottom of crucible12 that causes the melt to solidify from the bottom up, forcing anymagnesia trapped in the melt to rise to the top of the melt to join themagnesia already present at the top. At this point the coil iscompletely deenergized. After the entire melt has thus solidified andhas cooled to a suitable low level, the crucible is removed from chamber10 and its contants removed therefrom.

Thereafter the magnesia at the top of the ingot is removed by suitablemeans, e.g., by machining off the top of the ingot.

The resulting ingot has a high degree of freedom from oxygen, all of theoxygen in its bulk having been removed through its reaction withmagnesium to form magnesia which has floated to the top surface. Sincethe copper of the ingot is free of unreacted magnesium, its electricalconductivity has not been affected by the previous addition of magnesiumand is at the same high level as before the oxygen-removal process. Theamount of magnesia (MgO) left behind in the copper will be tooinsignificant to cause a deleterious effect in the electrode material ina vacuum interrupter.

From this ingot a suitable blank is cut, after which this blank ismachined into an electrode or contact of a vacuum-type circuitinterrupter. When the electrode has been mounted in the interrupter, theinterrupter is baked at a temperature of around 400 C and evacuated atthe same time to remove most of the surface contaminants from theelectrode. The bulk of the copper remains free of oxygen; and thus whenthe electrode is deeply eroded by an arc during interrupter operation,there is no release of oxygen from the copper matrix to interfere withinterrupting performance.

Considerable latitude is available in the amount of magnesium that canbe added in practising my process. The minimum amount has only to besufficient to combine with all the oxygen contained in the copper melt.An excess of magnesium based solely on stochiometric considerations isnecessary as the efficacy of the magnesium atoms to getter oxygen atomsis not 100 percent.

Taking as an example OF HC copper, which generally contains less than 2parts per million by weight of oxyen, e50 @199 xs=s s is normally 9Lis=nt hi$ takes into consideration the differ e ric e in the atomicweights of magnesium and oxygen, as well as the presence of surfaceoxygen. Thus, about 0.01 percent or 0.02 percent magnesium by weight isnormally sufficient. An excess of magnesium, even if quite large, willnot be deleterious other than in prolonging the time necessary forevaporating the uncombined magnesium. To maintain this time withinreasonable limits, I prefer to limit the percentage of magnesium to lessthan 0.1 percent by weight of the solution.

Free Energy of Oxide Formation A F, k cal/gram atom Oxide Be 0 136.3 Mg0 136.2 Zr 0 123.9 Ti 0 117.0

Thus, magnesiums affinity for oxygen approximately equals that ofberyllium and substantially exceeds that of zirconium and titanium.

A second advantage of magnesium when used in my process is that itsvapor pressure is much greater than that of copper. Accordingly, whenthe temperature is maintained at l,l50 C, the magnesium evaporates at amuch higher rate than the copper. Note in this respect that at 1,150 C,the vapor pressure of magnesium is about 1,000,000 times that of copper,as will be apparent from FIG. 2. ln marked contrast, the vapor pressureof beryllium at this temperature is approximately equal to that ofcopper, and the vapor pressure of titanium and of zirconium is muchlower than that of copper, all of which can be seen in FIG. 2. In orderto free the copper of any of these constituents, it would be necessaryto maintain the high temperature for a much longer time than is the casewith magnesium. This would not only prolong the time necessary toachieve the desired freedom of the gettering additive (thus increasingthe cost and decreasing the efficiency of the process) but also wouldcause much more of the copper to evaporate while the high temperature isbeing maintained.

Consideration was given to using aluminum as a gettering additive sinceits affinity for oxygen is only slightly less than that of magnesium,but (as seen in FIG. 2) its vapor pressure is so close to coppers in thepertinent temperature range that an unduly prolonged heating period isneeded to achieve the desired freedom from this additive.

The only other metals, aside from those mentioned hereinabove, whichhave a similarly high affinity for oxygen are thorium, yttrium, andhafnium. These, however, are not usable in practising my inventionbecause their respective vapor pressures at the pertinent temperature(i.e., about 1,150 C) are far lower than that of copper.

Magnesium is unique among all the gettering metals mentioned above inhaving not only a high affinity for oxygen but in having a vaporpressure at the pertinent temperature orders of magnitude greater thanthat of copper. This unique combination of properties renders magnesiumideal for use in my process, where after effective gettering action, anyuncombined residue thereof can be quickly and efficiently boiled off.

While 1 have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from myinvention in its broader aspects; and 1, therefore, intend herein tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by letters Patent of the UnitedStates is:

l. A method of preparing copper for use in arcing electrodes of avacuum-type circuit interrupter, comprising:

a. melting a quantity of copper in a vacuum,

b. adding to the molten copper a small amount of magnesium that reactswith any oxygen present in the copper to form magnesia, most of whichfloats to the surface of the liquid copper-magnesium solution,

c. maintaining while in a vacuum the temperature of the liquidcopper-magnesium solution at a level above the melting point of saidsolution and substantially above the boiling point of magnesium for asufficient period of time to evaporate from the solution all of themagnesium present therein that has not reacted with the oxygen therein,thus leaving a residue of liquid copper, and

d. forcing residual magnesia trapped in said liquid copper to move tothe surface of said liquid copper.

2. The method of claim 1 in which said residual magnesia trapped in saidliquid copper is forced to the surface thereof by directionally coolingand solidifying said copper.

3. The method of claim 1 in which the percentage of magnesium added tothe copper is below 0.1 percent by weight of the copper-magnesiumsolution.

4. The method of claim 1 in which the copper starting material referredto in (a) has an oxygen content of only several parts per million orless.

2. The method of claim 1 in which said residual magnesia trapped in saidliquid copper is forced to the surface thereof by direcTionally coolingand solidifying said copper.
 3. The method of claim 1 in which thepercentage of magnesium added to the copper is below 0.1 percent byweight of the copper-magnesium solution.
 4. The method of claim 1 inwhich the copper starting material referred to in (a) has an oxygencontent of only several parts per million or less.